1. Field
The present invention relates generally to data communication, and more specifically to techniques for performing successive interference cancellation (SIC) processing at a receiver of a multiple-input multiple-output (MIMO) communication system with multipath channels.
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on the other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NSxe2x89xa6min {NT, NR}. Each of the NS independent channels may also be referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
For a full-rank MIMO channel, with NS=NTxe2x89xa6NR, NT independent data streams may be processed to provide NT corresponding symbol streams, which may then be transmitted from the NT transmit antennas. The transmitted symbol streams may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different xe2x80x9creceivedxe2x80x9d signal-to-noise-and-interference ratios (SNRs) for a given amount of transmit power.
A successive interference cancellation (SIC) processing technique may be employed at the receiver to process NR received symbol streams from the NR receive antennas to recover the NT transmitted symbol streams. A SIC receiver successively processes the received symbol streams to recover one transmitted symbol stream at a time. For each stage of the SIC receiver, spatial or space-time processing is initially performed on the received symbol streams to provide a number of xe2x80x9cdetectedxe2x80x9d symbol streams, which are estimates of the transmitted symbol streams. One of the detected symbol streams is then selected for recovery, and this symbol stream is further processed to obtain a corresponding decoded data stream, which is an estimate of the transmitted data stream corresponding to the symbol stream being recovered. Each recovered symbol stream (i.e., each detected symbol stream that is processed to recover the transmitted data) is associated with a particular xe2x80x9cpost-detectionxe2x80x9d SNR, which is the SNR achieved after the spatial or space-time processing to separate out this symbol stream. With SIC processing, the post-detection SNR of each recovered symbol stream is dependent on that stream""s received SNR and the particular stage at which the symbol stream was recovered. The post-detection SNR of the recovered symbol stream determines the likelihood of correctly decoding the symbol stream to obtain the corresponding data stream.
To improve performance, the transmitted symbol streams need to be recovered in a particular order such that the likelihood of correctly recovering all transmitted symbol streams is maximized. This goal is made challenging for multipath channels that experience frequency selective fading, which is characterized by different amounts of attenuation across the system bandwidth. For a multipath channel, the post-detection SNR varies across the system bandwidth. In this case, some metrics other than post-detection SNR would need to be derived and used to select the order for recovering the transmitted symbol streams.
There is therefore a need in the art for techniques to process multiple received symbol streams, using SIC processing, to recover multiple transmitted symbol streams in a particular order such that improved performance may be attained.
Techniques are provided herein to process a number of received symbol streams in a MIMO system with multipath channels such that improved performance may be achieved when using successive interference cancellation (SIC) receiver processing. In an aspect, metrics indicative of the quality or xe2x80x9cgoodnessxe2x80x9d of a detected symbol stream are provided. These metrics consider the frequency selective response of the multipath channels used to transmit the symbol stream. In another aspect, techniques are provided to process the received symbol streams, using SIC processing, to recover a number of transmitted symbol streams. The order in which the transmitted symbol streams are recovered is determined based on the metrics determined for the detected symbol streams at each stage of the SIC processing.
In a specific embodiment, a method is provided for deriving a metric indicative of a received quality of a symbol stream transmitted via a multipath channel in a MIMO system. In accordance with the method, a post-detection SNR is estimated for each of a number of frequency bins used to transmit the symbol stream. The channel capacity of each frequency bin is then determined based on the post-detection SNR and an unconstrained or constrained capacity function. The channel capacities of all frequency bins are then accumulated to obtain an overall channel capacity. An equivalent SNR for an Additive White Gaussian Noise (AWGN) channel may also be determined based on the overall channel capacity and an inverse capacity function. The overall channel capacity and the equivalent SNR are both indicative of the received quality of the symbol stream. The metric may be used to select a specific symbol stream for recovery at each stage of a SIC receiver.
In another specific embodiment, a method is provided for processing a number of received symbol streams to recover a number of transmitted symbol streams in a MIMO system with multipath channels. In accordance with the method, the received symbol streams are processed to provide a number of detected symbol streams. A metric is then determined for each detected symbol stream. The metric considers the frequency selective response of one or more transmission channels (e.g., frequency bins) used to transmit the detected symbol stream, and may be defined as the overall channel capacity or the equivalent SNR. The detected symbol stream associated with the best metric is then selected for recovery (i.e., further processed to obtain a decoded data stream). A number of modified symbol streams, which have the estimated interference due to the recovered symbol stream approximately removed, may then be derived and further processed in similar manner to recover another symbol stream.
Various aspects and embodiments of the invention are described in further detail below. The invention further provides methods, processors, transmitter units, receiver units, base stations, terminals, systems, and other apparatuses and elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.